Furnace reversal system



p 19 70 J. STEWART, JR, ETAL 3,527,445

FURNACE REVERSAL SYSTEM Filed July 8, 1968 3 Sheets-Sheet 1 FIG. I

FUEL RATE RECORDER RELAY SYSTEM TEMPERATUREPH REVERSAL REVERSAL REVERSALFIG. in

I I l SET POINT I i I I TIME FUEL I FUEL FUEL ON OFF 0N OFF ON i OFFCOMPUTE COMPUTE Sept. 8, 1970 J. STEWART, JR, ETAL 3,527,445

FURNACE REVERSAL SYSTEM Filed July 8, 1968 I 3 Sheets-Sheet 2 TIME F 2 IFUEL-0N TIME TOTAL TIME PIT DRAWN Sept. 8,1970

J. STEWART, JR, ET AL FURNACE REVERSAL SYSTEM 3 Sheets-Sheet 5 TIME FIG.3 FUEL-0N TIME TOTAL TIME 100 90 l 70 so so 30 2o 10 0 J COVER OFF 3 TmgFIG. 4

FUEL-0N TIME TOTAL TIME COVER OFF United States Patent O 3,527,445FURNACE REVERSAL SYSTEM James Stewart, Jr., Philadelphia, Pa., andVincent J. Cucuzzella, Silver Spring, Md., assignors to Leeds & NorthrupCompany, Philadelphia, Pa., a corporation of Pennsylvania Filed July 8,1968, Ser. No. 743,176 Int. Cl. F27d 17/00 U.S. Cl. 263- 7 ClaimsABSTRACT OF THE DISCLOSURE In a regenerative furnace the reversal isactuated each time that the burners are turned off. This criteria forreversal is used when the signal representing the ratio of fuel on timeto total cycle time is below a preset, adjustable point. When the signalis above this point, the furnace is reversed when the burners have beenon for a given time interval. Uniform heating of the steel to thedesired temperature is indicated when an electrical signal representingthe rate of change of fuel flow to the furnace is substantiallyconstant.

BACKGROUND OF THE INVENTION Regenerative furnaces generally includechambers through which fuel, combustion air, and the products ofcombustion pass on the way to be ignited within the furnace. In orderthat maximum efficiency be obtained in the operation of the furnace, itis desirable to elevate the temperature of the combustion air beforecombustion Within the furnace. In order to do this, regenerativechambers at opposite ends of the furnace alternately receive heat fromthe products of combustion of the furnace and alternately give up theheat to the combustion air.

The operation of the furnace must be reversed periodically in order toprevent excessive cooling or overheating of the material of which theregenerative chambers are constructed. In the prior art, many techniqueshave been suggested for controlling the time at which the furnace isreversed. For example, U.S. Pat. 2,287,186 describes known techniquesfor determining the reversal time in regenerative furnaces. Of thesetechniques, perhaps the most straightforward is to reverse the furnaceafter definite time intervals.

Other, more complex techniques for determining the time of reversal aredescribed in U.S. Pats. 2,851,221, 2,177,805, 2,531,200 Davis, and2,804,268 Davis.

In a system in which the pit is reversed periodically, nothing preventsone side of the pit from carrying a dominant portion of the total firingtime. This can result in a large checker-brick temperature differentialbetween pit sides. Other of the prior art systems of the type de scribedabove provide a system which is sensitive to a temperature differentialin the chambers. Many of the systems of this type reverse at periodictime intervals unless there is an override caused by too great atemperature differential 'between the two sides. While systems usingtemperature differential provide a good safety factor, they do notbasically equalize the B.t.u. input to each side of the furnace. Thatis, any temperature measurement lags behind B.t.u. input so there isnever good equalization of B.t.u. input to both sides when 3,527,445Patented Sept. 8, 1970 this input is solely temperature control. Theplacement of the thermocouple also may not provide the average checkertemperature but only the temperature of a hot or cold area adjacent tothe thermocouple. All of the techniques described in the foregoingpatents have disadvantages which are obviated by the present inventionand which will become apparent from the following detailed description.

SUMMARY OF THE INVENTION This invention relates to an improvedregenerative furnace control system and to means for providing anindication of when the charge of steel is uniformly heated to thedesired temperature and ready to be removed from the furnaceAccordingly, it is an important object of the present invention toprovide improved temperature performance in regenerative furnaces byuniformly heating the charge and balancing regenerative checker-bricktemperature by controlling to an equal value the B.t.u.s fired from eachside.

It is another object of the present invention to reduce the totalheating cycle by reversing the pit while the fuel is off, rather than ona straight time basis which can reverse the furnace while the fuel ison.

It is another object of the present invention to provide an analogsignal of the rate of change of fuel flow to batch type heating furnacesand to provede, in response to this signal approaching a constant value,an indication that the charge is uniformly heated to a desiredtemperature and ready to be discharged from the furnace.

It is a further object of the present invention to produce a signalrepresenting the ratio of fuel on time to total cycle time, and toreverse the firing direction in response to the signal being above apreset, adjustable point, whenever the burner has been on for apredetermined time, and further to reverse the furnace, in response tothe signal being below the preset adjustable point whenever the fuel tothe furnace is cutoff.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of thefurnace reversal system of the present invention;

FIG. la shows furnace temperature as a function of time;

FIG. 2 shows the ratio of fuel on time to total time in a prior artsystem; and

FIGS. 3 and 4 show similar curves for a furnace of the presentinvention.

DESCRIPTION OF A PARTICULAR EMBODIMENT The invention in its preferredform has been shown applied to the control of a regenerative soaking pitfurnace 1 which has regenerative chambers or checkers, 2 and '3 at itsends. In the operation of the furnace, air is forced through an airinlet 4 into one of the checkers which preheats the air to supply heatedair for combustion. The furnace exhaust gases are forced out through theother checker to preheat that checker, and are exhausted through a stack5. A damper 6, located in the junction of the supply and exhaust pipes,directs air to the other end of the furnace and is rotated by to reversethe furnace operation. A suitable fuel is supplied to the furnacethrough a pipe 7 which has branches that lead to the burners 8 and 9,one of which is located in each end of the furnace in such a positionthat the incomming air can be mixed with the fuel thereby insuringcombustion. A reversing valve 10 operates to supply fuel to one or theother of the burners. The damper 6 and the valve 10 are actuated in aconventional manner to reverse the operation of the furnace.

What has been described thus far is a conventional regenerative typefurnace which further includes a set of relays 11 for effecting thereversal of the furnace. That is, when the relay system 11 is actuated,a sequence is initiated which reverses the position of the damper 6 andreverses the position of the valve 10.

In order to effect this reversal at the proper time in accordance withthe principles of this invention, a computing network is provided whichincludes integrating amplifiers 12 and 13 and the dividing, or ratio,network 14.

The computing network produces at the output of ratio network 14 asignal proportional to the ratio of the fuel on time to total timebetweeen reversals. This is referred to as the fuel rate signal.

The integrating amplifier 12 produces an output signal which isindicative of the time that either one of the burners 8 or 9 is on. Theburners 8 and 9 are temperature controlled. That is, a thermocouple 15measures the temperature in the furnace and a contact is actuated as thetemperature fluctuates about a set point. As shown in this particularembodiment, the contact 16 is included in the temperature recorder 17.The output of the thermocouple is applied to temperature recorder 17which has a balancing network of the conventional type. Through theaction of the balancing network, the pen on the recorder is moved up anddown scale in accordance with the temperature. At a predetermined pointin its travel, the contact 16 is opened and closed. This contact may beused to turn burners 8 and 9 on and off. Alternatively, actuation ofcontact 16 may merely be indicative of the time at which the burners 8and 9 are turned on and off.

When contact 16 is closed, indicating that one of the burners is firing,potential is applied to the integrating amplifier 12 which produces asan output a signal indicative of the time that the burner is on.Coincidental with the closure of the contact 16, the time 18 is started.That is, timer 18 is started each time that one of the burners is turnedon.

Assuming that the ratio of fuel on time to total time between reversalsis above a preset point, assume a 50% value, then the timer 18 will timeout and actuate the reversal relay system 11 after a given interval, say4 minutes.

The reversal relay system 11 requires about 20 seconds to complete areversal, that is, to go through the sequence of relay actuations whichwill effect reversal of the damper 6 and the valve 10. At the end ofthis reversal time, a relay contact is closed which applies a voltage tothe integrating amplifier 13.

Integrating amplifier 13 produces a ramp voltage output. At thebeginning of the next reversal period, the relay contact in system 11 isopened thereby disconnecting the input from integrating amplifier 13. Atthis time, integrating amplifier 13 has an output indicative of thetotal time between reversals. During the reversal time period (20seconds in the example being discussed), the computation of fuel rate isperformed. That is, the signal proportional to fuel on time fromintegrating amplifier 12 is divided by the signal proportional to timebetween reversals from integrating amplifier 13. This division isperformed in the dividing network 14 which produces the fuel ratesignal. During the reversal time period, the output of network 14 issampled and is applied to recorder 19 for recordation of a fuel ratevalue. Also, during the time interval in which a reversal is effected,the integrators 12 and 13 are reset so that they are ready for the nextcomputation. While the sample and reset functions for the computingnetwork have not been indicated in FIG. 1, the provision of thesefunctions at the times indicated is within the skill of the art.

When the fuel rate signal goes below a predetermined adjustable point(50% in the case under discussion), a different technique is used foreffecting reversals. The recorder 19 responds through the usualself-balancing network to position the pointer 20 to the properposition. At a predetermined point in its travel, in this case the pointindicating a 50% fuel rate, the mechanism associated with the pointer 20closes the normally open contact 22. That is, as the fuel rate fallsbelow the 50% point, the criteria for effecting reversals is changed.The reversal relay system 11 will no longer normally be actuated bytimer 18. Instead, the reversal relay system 11 is actuated each timethat the contact 16 is opened indicating that the burners are cut off.That is, each time that the furnace goes from fuel on to fuel off thepit will reverse.

The actuation of the relay reversal system under this condition may beunder what is referred to as automatic control or manual control. Inautomatic control, energizaton is through the circuit including contact16, closed contact 22 and contact 27a which is closed in automaticcontrol only. In manual control, energiiation is through the contact 16,through a contact 16a which is closed when the temperature isapproximately 20 below the set point, and through the contact 27 whichis closed only for manual control.

The operation of the furnace under these conditions is depicted in FIG.1a which shows furnace temperature as a function of time. FIG. la alsoshows the times at which the fuel is turned on and off and the time atwhich there is a reversal in the furnace. If, however, the fuel firesfor 4 minutes without turning off, this timer will initiate reversal asa safety override.

Included in the circuit between the reversal system and the integratingamplifier 13 is a cover interlock switch 28 which is opened when the pitcover is removed. The function of this interlock is to interrupt thecomputation for the period that the pit cover is removed. When the pitcover is again closed, the generation of the total signal timecontinues. This interlock is necessary because furnaces of this typehave another interlock which prevents the pit from firing when the coveris removed. If total time was allowed to accumulate while the firingcircuit was disabled, the recorded data during this period would be inerror.

Summarizing the operation of the system of FIG. 1, furnace reversal isregulated in response to two different criteria. When the signalrepresenting the ratio of fuel on time to total time is greater than50%, the reversal is controlled by the timer 18. When the timer 18 hasaccumulated a preset total of time that a burner has been under fire,then a reversal is effected. During this period, the furnace is on whatis termed high fire. When the necessary firing time has beenaccumulated, the furnace reverses, a computation is made, and the fueltime to total time ratio is recorded. The principal advantage of thistechnique of operation is that it insures that both sides of the pitwill receive an equal amount of fuel on time.

The second criterion of the pit reversal is applied during the period oftime when the ratio of fuel on time to total time is below 50%. Thechange from the first criterion to the second is accomplished byactuation of contact 22 in the recorder 19. When the second criterionfor pit reversal is employed, reversal of the furnace occurs every timethat the burners are turned off. That is, every time that the switch 16is actuated, the reversal relay system 11 is actuated.

As a further aspect of the present invention, there is provided anindicator 23 for indicating when the steel in the furnace is soaked out,that is, when the steel has uniformly reached the desired temperature.The basis for actuating this indicator is that as the soak out timenears, the ratio of fuel on time to total time approaches a constantvalue. As this time approaches, the burners are on merely for enoughtime to reesupply heat to the input which has been lost during the timethat the burners have been off. As the signal representing fuel rateapproaches a substantiallyconstant value, there is an indication thatsoak out has occurred. In order to determine the existence of such acondition, the signal representing fuel rate for one computation periodis compared to the signal representing fuel rate for the lastcomputation period. Note that the signal representing fuel rate for thelast computation period is available from the balancing slidewire 24 inthe recorder 19. This signal is applied to comparator amplifier 25.Also, the present value of the fuel rate signal, from dividing network14, is applied to the comparator amplifier 25. If the comparatoramplifier 25 indicates that the two signals are within a presettolerance, a counter 26 is advanced by one count. When the counter 26counts a selected number of cycles in which the previous fuel ratesignal was within the tolerance of the present fuel rate signal, thecounter 26 actuates the indicator light 23 to signal that soak out hasoccurred.

The advantages of the present invention over the prior art can be shownfrom FIGS. 24. FIG. 2 depicts a prior art type of operation in which thepit is reversed periodically. In this example, the pit was reversedevery 4 minutes. FIG. 2 is a plot of the ratio of fuel on time to totaltime as a function of time. The chart of FIG. 2 was made on a furnacewhich was reversed every 4 minutes. Computation of the fuel on time tototal time between reversals was made each time that the furnace wasreversed.

At the bottom of FIG. 2, in the portion of the record marked a, bothburners are firing 100% of the time. Since the pit is being reversedevery 4 minutes, there is no problem. Both burners are on equal amountsof time and both regenerative chambers will be equally heated. However,at the point 12, the firing ratio drops to approximately 60% of thetime. The left hand burner, which is being fired during this time, willbe on only approximately 60% of the time. However, at the point marked0, the burner on the right hand side is being fired approximately 96% ofthe time. The portion of the graph d shows the fuel on time afteranother reversal, that is, the fuel on time for the burner in the lefthand chamber. As the reversals are followed along the graph, it will beseen that the right hand burner consistently is being fired for agreater period of time than the left hand burner. This is a veryundesirable condition.

FIG. 3 shows the ratio of fuel on time to total time where the reversalis made each time the fuel is shut off. It can be seen that there are nowide variations in the record which means there was not a wide variationin the firing time from one burner to the other burner.

FIG. 4 is a chart similar to FIG. 3, except that the reversals have beenaveraged out. That is, computations have been made in which the computedvalues of heat rate have been averaged out for both combustion chambersrather than being computed separately for the left hand and the righthand combustion chamber. This is a desirable modification in order toget a better indication of when the steel is soaked out. Note that therelatively rapid variations in the chart of FIG. 3 make it difiicult toobtain consecutive readings within a narrow tolerance. However, when thevariations have been averaged out between the two burners as was done inobtaining the record of FIG. 4, it now becomes possible to obtain anumber of consecutive readings in which the fuel rate signal isconstant.

While a particular embodiment of the invention has been shown anddescribed, it will, of course, be understood that various modfiicationsmay be made without departing from the principles of the invention. Theappended claims are, therefore, intended to cover any such modificationwithin the true spirit and scope of the invention.

What is claimed is:

1. An apparatus for controlling the reversal of the flow of combustionair, and the products of combustion through a regenerative furnacehaving regenerative chambers at two ends and means to directalternatively the combustion air through said chambers and burners ineach of said chambers comprising in combination:

temperature responsive means for controlling the on and off condition ofthe firing of said burners in firing cycles,

timer means to determine the fuel on time during each firing cycle,means for producing a signal representing the ratio of fuel on time tototal cycle time, and

means operable when said signal is above a preset, ad-

justable, point for reversing the flow of combustion air and theproducts of combustion whenever said timer means exceeds a preset timeinterval, and operable when said signal is below a preset, adjustable,point to reverse the flow of combustion air and the products ofcombustion in response to a change in the firing condition of saidburners.

2. The apparatus recited in claim 1 wherein said means responsive to achange in the firing condition of said burners includes a relay contactactuated when said burners are cut off to reverse said flow ofcombustion air and the products of combustion.

3. An apparatus for controlling the reversal of the flow of combustionair, and the products of combustion through a regenerative furnacehaving regenerative chambers at two ends and means to directalternatively the combustion air through said chambers comprising incombination:

timer means to determine the fuel on time during each firing cycle,means for producing a signal representing the ratio of fuel on time tototal cycle time,

means responsive to said signal being above a preset,

adjustable, point for reversing the flow of combustion air and theproducts of combustion whenever said timer means exceeds a preset timeinterval, and

means responsive to said signal being below a preset,

adjustable, point for reversing said flow of combustion air and theproducts of combustion whenever the fuel to said furnace is cutoff.

4. The method of controlling the reversal of the flow of combustion airand the products of combustion through a regenerative furnacecomprising:

determining the ratio of fuel on time to total cycle time,

reversing the flow of combustion air and the products of combustionafter preset time intervals of burner firing whenever said ratio isabove a preset, adjustable, point, and

reversing the flow of combustion air and the products of combustion inresponse to said burner being turned off whenever said ratio is belowsaid preset, adjustable, point.

5. The method of indicating when the steel in a batch type heatingfurnace has been uniformly heated to the desired temperature comprising:

generating an electrical signal representing the rate of change of fuelflow to said furnace, and

actuating an indicating device when said signal approaches a constantvalue to indicate that the steel is uniformly heated to the desiredtemperature.

6. The method recited in claim 5 wherein said actuating step includes:

comparing said signal for a previous cycle with said signal for thepresent cycle,

actuating a counter each time the signal for the past and present cyclesare substantially equal, and energizing said indicating device when saidcounter has accumulated a predetermined number of counts.

7 7. The method of operating a regenerative furnace in which the flow ofcombustion air is alternatively directed through regenerative chamberscomprising: a I

generating an electrical signal representing the rate of change of fuelflow to said furnace, t I actuating an indicating device when saidsignal approaches a constant value to indicate that the steel in saidfurnace is uniformly heated to the desired temperature, v reversing theflow of combustion air and the products of combustion after preset timeintervals of burner firing whenever said signal is above a preset,adjustable, point, and i 8 t .reversingthe flow of combustion air andthe products I of combustion in response to said burner being i .turnedoff whenever said signal is below said preset,

adjustable, point;

References Cited UNITED STATES PATENTS 3,393,868 7/1968 'Griem 23615 vU.S. Cl. X.R.

